This invention relates to fluorescent polymers which are useful as water-insoluble indicators for the determination of the concentration of dissolved substances in an aqueous medium. The invention is also concerned with sensors comprising said fluorescent polymer indicators bonded to optical fibers.
The measurement in blood of pH levels and concentration of gases, particularly oxygen and carbon dioxide, is important during surgical procedures, post-operatively, and during hospitalization under intensive care and numerous devices for the measurement and display of said physiological parameters have been suggested in the art.
U.S. Pat. No. 4,003,707, Lubbers et al., and its reissue patent Re 31879, disclose a method and an arrangement for measuring the concentration of gases and the pH value of a sample, e.g. blood, involving the use of a fluorescent indicator enveloped by or sealingly embedded in a selectively permeable diffusion membrane. This patent discloses the use of beta-methyl-umbelliferone as an indicating substance for the measurement of pH values and pyrenebutyric acid as an indicating substance for the measurement of oxygen concentration.
The use of beta-methyl-umbelliferone as a fluorescent pH indicator was previously disclosed in an article by Raymond F. Chen, Analytical Letters 1[7], 423-428 [1968].
The use of pyrenebutyric acid as a fluorescent indicator for the determination of the concentration of oxygen in the blood also was known before the Lubbers et al. patent, as disclosed in U.S. Pat. No. 4,041,932, Fostick. Fostick discloses a method whereby blood constituents are monitored by measuring the concentration of gases or fluids collected in an enclosed chamber sealingly attached to a skin "window" formed by removing the stratum corneum over a small area of the patient's skin. The measurements in the enclosed chamber are made, inter alia, by determining the difference in intensity of light emitted from a fluorescent indicator.
The use of a pH sensitive dye indicator in conjunction with a fiber optic pH probe is disclosed in U.S. Pat. No. 4,200,110, Peterson et al. In U.S. Pat. No. 4,476,870, Peterson et al. disclose a similar technique utilizing dye fluorescence oxygen quenching. In both Peterson et al. patents the relevant fluorescent dye indicator composition is enclosed within a selectively permeable membrane envelope.
An apparatus for measuring oxygen concentration based on oxygen quenching of molecular luminescence is disclosed in U.S. Pat. No. 3,612,866, Stevens.
A sensor adapted to function satisfactorily in a biological environment should possess at least four characteristics: sensitivity, short response time, stability and bio-inertness.
Sensitivity depends upon the quantum efficiency of the fluorescent indicator, the concentration of the indicator present in the sensor and availability of the indicator to the substance, i.e. ion or gas, it must sense. Thus a sufficient amount of indicator must be available to produce a meaningful fluorescent response. However, if indicator molecules are too close together there occurs a type of behavior which is frequently detrimental to the sensor performance; this behavior is known as eximer fluorescence. Therefore, for a given indication there is an optimum indicator concentration for maximum sensitivity.
A further problem which must be solved in the construction of a fluorescent sensor is the availability of the indicator to the environment to be sensed. If the subject ions or gas cannot reach the indicator molecules the indicator will not respond to the presence or absence of said ions or gas. This problem is clearly related to the permeability of the structure in which the indicator molecules are embedded.
Also related to said permeability is the question of response time. If the substance to be sensed (i.e. ions or gas) diffuses very slowly through the structure the response time of the sensor will be comparatively long which greatly reduces its usefulness.
A sensor for blood gas or blood pH should be capable of use over a period of many hours or days. Recalibration of a sensor which is used in vivo is clumsy and inefficient or even impossible. Thus, the stability of the sensor is a key factor in determining its utility. A common problem in existing fluorescent sensor design is the gradual loss of the indicator from the sensor. This not only reduces the sensitivity, thereby creating instability in the sensor's indication even at constant concentration of the substance being sensed but also releases a chemical indicator into the blood stream. A device which releases chemical substances into the blood stream can not be considered to be bio-inert. As used herein, the term "bioinertness" is defined to mean that characteristic of a device, i.e. a sensor, whereby any and all chemical substances which are part of the device are so securely bonded to the structure of the device that they are not released or leached away from the device under normal operating conditions.
In each of the above prior art disclosures the problem of leaching of the indicating substance from the sensor, which is inherent when small molecules are embedded in a polymer matrix, was addressed by enveloping or embedding the indicator in a selectively permeable membrane.
In practice, said problem manifests itself as a progressive loss of sensitivity of the sensor as the indicator is lost; this requires a continual re-calibration of the sensor.
The stated prior art arrangement does not completely solve the problem, since a portion of the indicating substance is still leached from the sensor. Thus, the problem of re-calibration still remains, and, moreover, the released indicator goes into the patients blood-stream.
Accordingly, there is a definite need in the art for a sensor which is more stable in the sense that the indicator is not leached or washed away therefrom upon contact with body fluids.
It has now been found that the desired stability may be achieved if the indicator is chemically bonded to an appropriate polymer so that the resulting chemical entity may be contacted directly with the aqueous medium under examination without loss of the indicating substance. At the same time the bonding of the indicator to the polymer does not result in any reduction of the rapid response characteristics possessed by the free indicating substance. U.S. Pat. No. 4,194,877, Peterson, which is a companion case to the above-mentioned U.S. Pat. No. 4,200,110, discloses a hydrophilic polymer composition of acrylamide and a dye material selected from the group consisting of phenol red, brilliant yellow and rosolic acid. However, it is clear from the chemical structure of these particular dyes that they do not possess the necessary reactive sites to chemically combine with a polymer to form a chemical entity comparable with the novel fluorescent polymers of the present invention.
U.S. Pat. No. 3,904,373, Harper, discloses an insolubilized bound indicator consisting of an organic indicator covalently coupled by means of an organo-functional silane coupling agent to an inorganic carrier, for example a glass.